62 COLLEGE ZOOLOGY 



of these increase in size and develop into macronuclei (I)', the 

 other four remain micronuclei. The whole animal then divides 

 by binary fission (m, n), each daughter cell securing two of the 

 macronuclei and two micronuclei (o). Another binary division 

 (p) results in four cells each with one macronucleus and one 

 micronucleus (q). An indefinite number of generations are 

 produced by the transverse division of the four daughter cells 

 resulting from each conjugant. 



The significance of conjugation cannot be definitely stated. 

 Some investigators believe that Paramecium passes through a 

 life cycle containing three distinct stages. The period of (i) 

 youth is characterized by rapid cell multiplication and growth; 

 (2) maturity by less frequent cell division, sexual maturity, and 

 the cessation of growth; and (3) old age by degeneration and 

 natural death. Death is avoided by conjugation, which rejuve- 

 nates the senescent animals. 



Jennings has shown that some Paramecia conjugate more 

 often than others, and Woodruff has succeeded in carrying a cul- 

 ture through a period of over five and one half years. During 

 this time there were over three thousand and five hundred 

 generations. These facts " weaken the theory that conjugation 

 is to be considered the result of senile degeneration at the end of 

 the life cycle," and show that this Protozoon "has unlimited 

 power of reproduction without conjugation or artificial stimula- 

 tion " if given a favorable environment. 



b. Infusoria in General 



The Infusoria are Protozoa with cilia which serve as loco- 

 motor organs and for procuring food. Paramecium is a typical 

 member of the class. There are two subclasses, (1) CixiATAand 

 (2) Suctoria. 



Subclass i. Ciliata. — Infusoria with cilia in the adult 

 stage, a mouth, and usually undulating membranes or cirri. 

 Many ciliates are confined to fresh water, others occur either in 

 fresh or salt water, and still others are parasitic in Metazoa. 



